1) Field of the Invention
The present invention relates to a filament-reinforced resinous structural rod. More particularly, the present invention relates to a filament-reinforced resinous structural rod able to be used as a reinforcing member for a structural material made of a concrete or plastic resin material, to thereby enhance the mechanical strength of the structural material.
2) Description of the Related Arts
A conventional fiber-reinforced plastic resin rod (referred to as FRP rods hereinafter) in which the tensile strength is reinforced in one direction by fibers, is known and utilized for various purposes, and when used in place of an iron or high strength steel reinforcing rod for reinforcing a concrete structure, the FRP rod is advantageous in that it has a very high corrosion resistance, and thus the thickness of a concrete layer around the reinforcing rod can be made thinner even under highly corrosive conditions, and has a small specific gravity corresponding to about 1/6 of that of a steel material, and thus allows a reduction of the weight of the resultant reinforced structure.
Also, when used as a stretching or tensing material for a prestressed concrete structure, the FRP rod has a smaller Young's modulus than that of a reinforcing steel material, and thus can be utilized at a lower stress loss than the steel material and can avoid breakages of the stretching material due to stress corrosion.
Research into the conventional stretching materials consisting of the FRP rod was carried out in the U.S.A., the United Kingdom, and the U.S.S.R. from 1950 to 1960, but these conventional types of stretching materials are disadvantageous in that, when a tensile load is continuously applied, the degrees of tensile stresses created on individual reinforcing fibers in the FRP rod-reinforced structure are not even and the tensile strength of the FRP rod is lowered with a lapse of time, i.e., static fatigue occurs in the FRP rod. Therefore, the above-mentioned research was temporarily abandoned.
Recently, however, a new pultrusion method was developed in which reinforcing fibers are bundled to form rods having a uniform cross-section by applying a uniform tension thereto, and as a result, the reduction in the tensile load due to static fatigue was greatly lowered. Accordingly, the utilization of the FRP rod as a stretching material having a non-magnetic property is now under investigation.
When the FRP rod is employed as a reinforcing material or a stretching material for concrete structure, it is very important that the FRP rod has a high bonding property to concrete. Nevertheless, the surface of the FRP rod is coated with a resinous material, and therefore, cannot be firmly bonded to concrete with a chemical bonding agent. Accordingly, the bonding property of the FRP rod to concrete has been enhanced by introducing a twisted wire structure or a braid structure into the FRP rod, to form the FRP rod into an irregular shape. This irregular shape of the FRP rod has a rugged periphery which exhibits an anchoring effect when bonding the FRP rod to concrete.
Japanese Unexamined Patent Publication No. 61-28092 (prior art 1) discloses an FRP rod having a twisted wire structure, and this twisted wire structure type of FRP rod is produced by impregnating a filament core element having a high tensile strength and a low ultimate elongation with a thermosetting resin; applying a dry inorganic powder to the periphery of the filament core element; covering the dry inorganic powder-applied periphery of the filament core element with a knitted or braided fibrous layer; and thermosetting the resin by heating.
Japanese Unexamined Patent Publication No. 60-119853 (prior art 2) discloses a FRP rod having a braid structure, and this braid structure type of FRP rod is produced by braiding a plurality of fine threads having a high tensile strength.
The above-mentioned prior arts 1 and 2 very effectively enhance the resistance of the FRP rod to tensile stress, but the individual reinforcing fibers in the FRP rod are not always extended along the longitudinal direction of the FRP rod, and therefore, the tensile strengths of the individual reinforcing fibers are not sufficiently utilized in the longitudinal direction of the FRP rod, and thus the tensile stresses created in the individual reinforcing fibers are not even and an uneven static fatigue is generated in the individual reinforcing fibers. This phenomenon makes the irregularly formed FRP rod disadvantageous when employed as a stretching material for a prestressed concrete structure.
In view of the above-mentioned prior arts, preferably a new type of FRP rod, in which the tensile strengths of the individual reinforcing fibers are effectively utilized in the longitudinal direction of the FRP rod, is prepared by a pultrusion method so that the individual reinforcing fibers are arranged substantially in parallel to each other in the longitudinal direction of the FRP rod, and the resultant FRP rod has a rugged periphery thereof.
This type of new technique is disclosed in Japanese Unexamined Patent Publication No. 61-274036 (prior art 3). In this type of FRP rod, a continuous filament is spirally wound around a cylinder-shaped continuous filament bundle (core) in which a number of continuous filaments are arranged substantially in parallel to each other in the longitudinal direction of the FRP rod, and bonded to each other through a thermosetting resin, to provide an FRP rod having a rugged periphery thereof.
The technique of prior art 3 is excellent in that the mechanical strength of the individual filaments can be effectively utilized in the longitudinal direction of the FRP rod and the rugged periphery can be formed by employing a small amount of the filaments. Nevertheless, this type of FRP rod is still disadvantageous in that the binding force of the spirally wound filament to the core is not satisfactory, and thus the bonding force of the resultant FRP rod to concrete is not satisfactory. Namely, when a tensile stress is applied to the FRP rod embedded in concrete, the spirally wound filament is separated from the core, and therefore, the FRP rod is not firmly bonded to the concrete. Particularly, when the spirally wound filament consists of an aramid filament, which per se has a relatively low interfacial bonding property to the thermosetting resin, the above-mentioned poor bonding tendency is worsened.
Accordingly, it is necessary to provide a new type of FRP rod in which the spirally wound filament is not easily separated or peeled from the core.